Series resistor-condenser starter for a transistorized oscillator



1366- 1965 D. F. CAMPBELL 3,222,583

SERIES RESISTOR-CONDENSER STARTER FOR A TRANSISTORIZED OSCILLATOR Filed Dec. 5, 1960 g 8 I 8 1| l v k w "a: 5 r0 8 co n 1| 1| II g E g g g a g N g 2 s IN V EN TOR. DAVID F. CAMPBELL ATTOR YS United States Patent 3,222,588 SERIES RESISTOR-CONDENSER STARTER FUR A TRANSISTORIZED QSCILLATOR David F. Campbell, Sidney, N.Y., assignor to The Bendix Corporation, Sidney, N .Y., a corporation of Delaware Filed Dec. 5, 1966, Ser. No. 73,623 7 Claims. (Cl. 320-1) This invention relates to a novel apparatus for generating surges of electric current. In the described illustrative embodiment of the invention, the apparatus is useful for the generation of a continuous series of surges of electric current, as in an ignition system for use with an engine of the jet type.

The invention has among its objects the provision of novel apparatus for generating surges of electric current.

A further object of the invention lies in the provision of apparatus of the type indicated which employs a novel circuit employing transistors as a source of electric pulses for generating the desired surges of electric current.

A still further object of the invention, in specific embodiments thereof, lies in the provision of apparatus of the type indicated employing a direct current source of power and a square wave generator, the square wave generator having transistorized circuits for generating pulses of electric current which power the square Wave generator.

Yet another object of the invention lies in the provision of an improved ignition circuit which will operate satisfactorily despite wide changes in the voltage of the electric power source therefor, and which is capable of successful operation in a wide temperature range such as would be encountered by its operation in conjunction with a jet engine in either Arctic or tropical climates.

The above and further objects and novel features of the invention will more fully appear from the following description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for the purpose of illustration only, and is not intended as a definition of the limits of the invention.

In the drawing, wherein like reference characters refer to like parts,

The sole figure of the drawing is a schematic wiring diagram of an ignition system for an engine of the jet type, such ignition system incorporating the improved apparatus for generating surges of electric current in accordance with the present invention.

The circuit shown in the drawing may be considered as generally consisting of three main parts: a power source and filter designated 10, a square wave generating portion 11, and an output portion, which in this instance has two generally similar output circuits 14 and 14 connected in parallel. Portion 11 of the circuit includes an output transformer 12 upon the primary winding of which there is impressed an alternating current voltage of square wave configuration. The parallel output or capacitor discharge circuits 14 and 14 are simultaneously energized from transformer 12 through separate full wave voltage doubler arrangements. The circuit of the invention is such that should one of circuits 14, 14 cease to operate, as for example by reason of the short circuiting of its storage capacitor, the other discharge circuit continues to operate.

The circuit is powered by a battery (not shown), the positive terminal of which is connected to terminal 15 of circuit portion 10, and the negative terminal of which is connected to ground through terminal 16. Terminal 15 is connected, as shown, to a conventional radio filter including a series connected coil across which two condensers are connected in shunt. The midpoint of the Wire connecting the condensers is grounded. The output of the filter is connected by a wire 17 to an input terminal 3,222,588 Patented Dec. 7, 1965 67 of the circuit portion 11. Circuit portion 11 is powered by a drive transformer 19 which has one primary 20, a magnetic core 21, and four secondaries 22, 24, 25, and 26 inductively associated with the primary and the core. Transformer 19 is of the saturating type, the flux in the core 21 rising to a peak value in either direction depending upon the direction of excitation of primary 20. Four sub-circuits powered by the respective transformer secondaries are employed. Sub-circuits 27 and 27', which are similar but for the reverse connection of their transistors and 30, are connected to the secondaries 22 and 25, respectively. Parts in sub-circuit 27, which are the same as those in circuit 27, are designated by the same reference characters with an added prime. Sub-circuits 29 and 29 are connected to secondaries 24 and 26, respectively. These two sub-circuits are similar, except for the reverse connection of their transistors 45 and 45'. Elements in circuit 29' which are the same as those in circuit 29 are designated by the same reference characters with an added prime.

The transistor 30 of sub-circuit 27 has a base 31 which is connected by a wire 32 to one end of secondary 22. The transistor 30 has a collector 34 which is connected to ground through wires 35 and 36. The other end of wire 36 has a condenser 37 and a resistor 40 disposed in series therein, condenser 37 being shunted by a further resistor 39. The second end of resistor 40 is connected to the other end of the secondary 22 by a wire 41. A resistor 42 is interposed in wire 44 beyond its connection to wire 41.

In subcircuit 27' the base 31' of transistor 30 is directly connected to one end of secondary 25 by a Wire 32'. A wire 44' leading from the other end of secondary 25 has a resistor 42 interposed therein. Connected to wire 44 at a point between resistor 42 and secondary 25 is a wire 41 which is connected to ground through series connected resistor 40', condenser 37', and wire 36'. Condenser 37 is shunted by a resistor 39'.

In sub-circuit 29 there is a third transistor 45 which has its base 46 connected to one end of secondary 24 by a wire 47 and a series connected resistor 49. The other end of secondary 24 is connected by a wire to a wire 56 to which the above-described terminal 67 and wire 44' are connected.

In sub-circuit 29' the base 46 of transistor 45' is connected to one end of secondary 26 by a wire 47 and a series connected resistor 49'. The other end of secondary 26 is connected to a lead wire 50'.

The four sub-circuits are interconnected as shown. The emitter 51 of transistor 30 of sub-circuit 27 is connected by a wire 52 to the collector 54 of transistor 45 of subcircuit 29. Wire 52 is connected at junction C to wire 44 which is connected to one end of primary 75 of output transformer 12. The emitter 55 of transistor 45 of subcircuit 29 is connected by wire 56 to the emitter 51 of transistor 30 of sub-circuit 27'. Wire 56, as we have seen above, is also connected to wires 44 and 50 and to terminal 67. The collector 34' of transistor 30' of subcircuit 27' is connected by a wire 35' to the emitter 55 of transistor 45' of sub-circuit 29. Wire 35' is connected at junction B to wire 50 and is connected through a series connected winding 72 of a choke coil 71 having a magnetic core 74 to the other end of the primary 75 of output transformer 12. The collector 54' of transistor 45 is connected to ground by wire 62. The primary 20 of drive transformer 19 has one end thereof connected to a terminal 69 on wire 44 through a wire 70. The other end of primary 20 is connected to a terminal 66 on wire 50 by a wire 64 having a resistor interposed therein.

It will be apparent from the above and from the description of the manner of operation of portion 11 of the circuit, set out below, that in one portion of the operating cycle transistors 30 and 3% are conductive whereas transistors 45 and 45 are nonconductive, and that in the further, following portion of the cycle transistors 30 and 30 are nonconducting Whereas transistors 45 and 45' are conducting. As a result, the primary 75 of power transformer 12 is subjected to a series of electric pulses of alternating polarity. Such pulses, which form in effect an alternating current of square wave form, induce high voltage alternating current in the secondary 77 of the transformer which is disposed in inductive relationship with the primary 75 and the magnetic core 76 thereof.

The discharge circuits 14 and 14' are connected in parallel to the secondary 77 of the power transformer. Elements of circuit 14' which are the same as those in circuit 14- are designated by the same reference characters with an added prime Thus one wire 79 leads directly to circuit portion 14, a further wire 80 connected to wire 79 leading to circuit portion 14. Wire 79 connects through a coupling condenser 81 and a first halfwave rectifier 82 to a wire 84 leading to one terminal of a control gap 37. The other end of secondary 77 is connected to ground through a wire 85. A second, similarly disposed half-wave rectifier 83 and a series connected, current-limiting resistor 88 are connected between wire 85 and the output terminal of coupling condenser 81. A storage condenser 86 is connected in shunt between wires 84 and 85, there being a resistor 92 shunted across condenser 85. The second terminal of control gap 87 is connected by a wire 93 to a discharge device 90 which is connected with its terminals between wires 85 and 93. Device 9i) may be of the shunted gap type having a resistor 91 connected between electrodes.

Discharge circuit 14 functions progressively to charge the storage condenser 86 upon each positive pulse in secondary 77. When the charge on the storage condenser has reached a predetermined voltage, the control gap 87 breaks down so as to discharge a surge of current across the gap of the discharge device 9th.

Circuit 14' is identical with circuit 14, except for the fact that the half-wave rectifiers 82 and 83' are disposed in the circuit in a manner which is the reverse of rectifiers 82 and 83. Consequently, the storage condenser 86' of circuit 14 is progressively charged only by the negative portions of the alternating voltage generated in secondary 77 of the power transformer. With the rectifiers connected as shown, capacitor 81' assumes a charge at the same time storage capacitor 86 is receiving a charge and capacitor 81 is charged simultaneously with storage capacitor 36. ously energized, and the load for each voltage polarity at transformer 12 is substantially equalized. The connection between the circuits 14 and 14' is such that should one of them become inoperative the other continues to function. Thus the described circuit provides a valuable safety factor when both discharge devices 90 and 99 are employed with the same jet engine.

It will be seen from the above that in the portion 11 of the circuit the primary 2t) of drive transformer 19 is mechanically speaking connected in a closed loop with choke coil 72 and the primary 75 of output transformer 12. It will also be seen that the wire 17 from the direct current source is alternately connected to different sides of such loop, that is, at one time being connected between primary 2t) and primary 75 and in the other portion of the cycle being connected between primary 2th and coil '72. At the same time, the opposite points of connection to the loop are alternately connected to ground. Thus, electrically speaking, primary winding 20 and windings 72, 75, are connected in parallel circuits across power source 1tl through the conductive pairs of transistors. This causes the described current pulses in opposite directions in primray 75 of output transformer 12, so that a square wave voltage with a peak value of twice the battery voltage appears across primary 75. The described Thus, the circuits 14 and 14 are simultaneswitching of the polarity of the battery source is efiected at a first part of the cycle by the transistor 39 and 39' which are then conductive to connect opposite sides of the loop to ground and to the battery, respectively. In the next portion of the cycle transistors 23% and 3d are rendered nonconductive, and transistors 4-5 and conductive. Under such conditions transistor 45 then connects the junction 66 of the loop to ground and transistor 45 connects junction 69 of the loop to the battery.

It will be seen that in the described circuit the transistors 3d, 45, 3t), and 35 are, in effect, connected in a closed loop or bridge, since the wires 36 and s2 are, in effect, connected to ether at the ground A. Such bridge circuit has first, second, third, and fourth connection points B, 67, C, and ground A. A first transistor 30' is connected between the first point B and second point 67. A second transistor 45 is connected between the second point 67 and the third point C. A third transistor 30 is connected between the third point C and the fourth point, ground A. A fourth transistor is connected between the fourth point, ground A, and the first point B. T he feedback transformer 19 has its primary winding 26 connected across the first point B and the third point C. Transformer 19 has first, second, third, and fourth secondary windings 25, 24, 22, and 26, respectively, each of the transistors having its base electrode connected to the corresponding secondary winding.

The base electrodes of transistors 34) and 30 are connected to the ends of their windings corresponding to a given polarity for a voltage in said primary winding 20 in a given direction, the base electrodes of transistors 45 and 45 being connected to the ends of their secondary windings corresponding to the opposite polarity. The source of direct current 15, 16 is connected between the second and fourth points; the primary winding of power transformer 12 is connected across the first and third points B and C, respectively. The thus described bridge circuit oscillates to provide a square wave output with the first transistor 3d and the third transistor 30 biased to conduction only during one-half of the oscillating cycle, the second and fourth transistors 45 and 45', respectively, being biased to conduction only during the other half of the oscillating cycle.

The manner of operation of the circuit will be more fully understood from the following detailed description of its operation during one complete cycle of the application of a square wave voltage on the primary 75 of transformer 12. In such description it will be assumed that the battery employed as a direct current source delivers 14 volts. Upon the initial closing of the circuit from the battery through the filter to the circuit 11, terminal 67 will be at a positive potential of 14 volts. The resistors 42' and .1) form a voltage divider with respect to ground, since capacitor 37' is initially uncharged and its charge can not be altered instantaneously. Therefore, there will be a small voltage drop, which may be, for example, on the order of 0.5 volt with typical circuit parameters, across resistor 42'. Such small voltage drop causes the emitter 51' to have a potential greater than that of the base 31' of transistor 30, thus causing a current to flow from emitter 51 to base 31'. Such emitter-base current causes transistor St? to turn on or become conductive, so that the battery voltage will now appear at junction 66. Since transistor '30 has not as yet been turned on, the battery voltage also appears at 69. The same sequence of electrical events which cause transistor 30 to turn on will be repeated, and the current flow between emitter 51 and base 31 of transistor 30 will cause transistor 30 to turn on or to become conductive between its emitter 51 and its collector 34. When this occurs, junction 69 will be connected directly to ground through wires 35 and 36, except for a very small voltage drop across the transistor itself.

With both transistors 3d and 36 now conducting, the major portion of the battery voltage will appear between junctions 66 and 69 or, in other words, across the primary of driving transformer 19 and across the primary 75 of output transformer 12 in series with choke coil winding 72. With the battery voltage now applied to the primary 20 of transformer 19 between junctions 66 and 69, driving transformer 19 continues to supply the necessary emitter-base currents which keep transistors 30 and 3t) fully turned on for a period of time governed by the design of transformer 19.

Transformer 19 is, as above described, of the saturating type. When the core 21 of transformer 19 has reached a saturated condition, the secondary windings 22 and 25 of the transformer can no longer supply the voltages necessary to maintain the secondary currents between the emitters and the bases of the respective transistors and 30'. This causes transistors 30 and 30' to be turned off or become non-conductive. Simultaneously with the start of the operation of turning off of transistors 30 and 30', the action of the transformer 19 causes the polarity of each of the secondary windings 22, 24, 25, and 26 to reverse as the flux of core 21 begins to return to its residual value. This action not only insures that transistors 30 and 30' will be rendered non-conductive very rapidly, but also causes transistors 45 and 45' to be turned on or made conductive by setting up emitter-base currents in such transistors.

With transistors 45 and 45' in conductive condition, the battery voltage will have been reversed with respect to the loop 20, '70, 75, 72, 65, and 64. Thus junction 66 will now be connected to ground through transistor 45', and junction 69 will be connected to battery voltage through transistor 45. This portion of the cycle continues until core 21 of transformer 19 again becomes saturated, at which time transistors 45 and 45 will be rendered nonconductive, and transistors 30 and 30' will again be rendered conductive.

It has been found that the described circuit for powering the primary '75 of power transformer 12 is particularly Well adapted for use with the ignition circuit shown and described. Each of the output portions 14 and 14' of such circuit includes a storage condenser, 86 and 86', rerespectively. When 'a condenser is in fully discharged condition, it constitutes substantially a short circuit at the start of its charging cycle. Thus the current demands made upon transformer 12 and the transistor bridge circuit supplying such transformer would vary so widely as to prohibit the practical commercial use of such bridge circuit if it were not for the provision of the currentlimiting choke coil 71 interposed in series with the primary 75. Such choke coil limits the initial rush of current into primary 75 when the condensers 86 and 86' are in fully discharged position, while still permitting adequate current flow thereinto to permit the charging of condensers 86 and 86' within the required portion of the operating cycle.

Because of the markedly fluctuating current demand of the output circuit or circuits, a transistor bridge circuit supplying the demands of such output circuit or circuits is particularly hard to start into oscillation. It is for this reason that the novel starting means provided for each of sub-circuits 27 and 27' in the transistor bridge circuit shown have proved to be particularly effective. Such starting means insures that the transistors 30 and 30 will be initially rendered conductive when the circuit is placed in operation, and that the bridge will oscillate, despite subjection of the circuit and its components to wide variations in operating conditions, as is required for reliable operation of ignition systems for jet aircraft and the like.

Although only one embodiment of the invention has been illustrated in the accompanying drawing and described in the foregoing specification, it is to be especially understood that various changes, such as in the relative dimensions of the parts, materials used, and the like, as well as the suggested manner of use of the apparatus of the invention, may be made therein without departing from the spirit and scope of the invention as will now be apparent to those skilled in the art.

What is claimed is:

1. A circuit for generating current pulses comprising a transistor having a base, an emitter, and a collector, a first circuit connecting the emitter to the base, a first resistance interposed in the first circuit, a second circuit connected to the first circuit between the base and one end of the first resistance, said second circuit extending to a common conductor and having a series connected second resistance and capacitor interposed therein, and a source of direct current having its positive terminal connected to the emitter and the other end of the first resistor and its negative terminal connected to the common conductor, whereby the first and second circuits and the resistances therein constitute a voltage divider which biases the emitter with respect to the base to render the transistor conductive between its emitter and collector.

2. A bridge coupled transistor oscillator comprising an intercoupled feedback transformer and four transistors serially conductive in pairs, different pairs being alternately conductive, a source of electrical energy connected to supply electrical energy to the primary winding of said transformer through the emitter-collector path of the conductive pair of transistors, a feedback loop circuit for each transistor including a secondary winding of said transformer, a resistor and the emitter and base of the respective transistor, and a starting circuit for each transistor of only one of said pairs comprising a resistor and a capacitor connected in series between said source and said loop circuit.

3. Electrical apparatus comprising a transistor having base, emitter and collector electrodes, a feedback transformer having primary and secondary windings, biasing circuit means comprising said secondary winding and a resistor connected in series across said base and emitter electrodes, means connecting said primary winding across a source of electrical energy through said emitter and collector electrodes, and a resistor and a capacitor connected in series between said biasing circuit means and a terminal of said source.

4. In an oscillator, first and second current paths, switching means cooperating with said current paths for establishing periodically a current conductive condition of the first path and a current blocking condition of the second path followed by a current blocking condition of the first path and a current conductive condition of the second path, said switching means comprising a transistor in each of said current paths, a base electrode biasing circuit for each transistor comprising the secondary winding of a feedback transformer and resistor connected in series across the emitter and base electrodes of the respective transistor, and a starting circuit for only one of the transistors comprising a capacitor and a resistor connected in series between a source of electrical energy for said current paths and the base biasing circuit for said one transistor.

5 A self-excited transistor oscillator comprising a transistor having a base, an emitter and a collector, means connecting the emitter to one terminal of a source of electrical energy, a feedback transformer, a first resistor connected in series with the secondary winding of said transformer across said base and emitter, a capacitor and a second resistor in series, means connecting said first resistor in series with said capacitor and said second resistor across said source, and a load circuit including the primary winding of an output transformer connected in series with said emitter and collector across said source, said load circuit further comprising a choke coil in series with the primary winding of the output transformer, and said feedback transformer having a primary winding connected in parallel with the series connected choke coil and primary winding of the output transformer.

6. A transistor oscillator comprising a transistor having a base, an emitter and a collector, means connecting the emitter to one terminal of a source of electrical energy, a feedback transformer, a base biasing circuit comprising a first resistor connected in series with said emitter and base across the secondary winding of the feedback transformer, a capacitor and a second resistor connected in series, and means connecting said series connected capacitor and second resistor between the other terminal of said source and to a point on said biasing circuit between said first resistor and said base.

7. A circuit for generating electric current pulses comprising a bridge coupled transistor oscillator including an intercoupled feedback transformer and four transistors serially conductive in pairs, diiferent pairs being alternately conductive, a source of electrical energy connected to supply electrical energy to the primary inding of said feedback transformer through the emitter-collector path of the conductive pair of transistors, a feedback loop circuit for each transistor including a secondary winding of said transformer and the emitter and base of the respective transistor, an output power transformer including a primary winding connected in series with a choke coil, said series connected winding and coil being connected in parallel with said primary winding of the feedback transformer, and an output circuit including the secondary Winding of said power transformer, a storage condenser connected to be charged by said secondary winding of the power transformer and means for intermittently discharging said storage condenser.

References Cited by the Examiner UNITED STATES PATENTS GEORGE N. WESTBY, Primary Examiner.

SAMUEL BERNSTEIN, Examiner. 

7. A CIRCUIT FOR GENERATING ELECTRIC CURRENT PULSES COMPRISING A BRIDGE COUPLED TRANSISTOR OSCILLATOR INCLUDING AN INTERCOUPLED FEEDBACK TRANSFORMER AND FOUR TRANSISTORS SERIALLY CONDUCTIVE IN PAIRS, DIFFERENT PAIRS BEING ALTERNATELY CONDUCTIVE, A SOURCE OF ELECTRICAL ENERGY CONNECTED TO SUPPLY ELECTRICAL ENERGY TO THE PRIMARY INDING OF SAID FEEDBACK TRANSFORMER THROUGH THE EMITTER-COLLECTOR PATH OF THE CONDUCTIVE PAIR OF TRANSISTORS, A FEEDBACK LOOP CIRCUIT FOR EACH TRANSISTOR INCLUDING A SECONDARY WINDING OF SAID TRANSFORMER AND THE EMITTER AND BASE OF THE RESPECTIVE TRANSISTOR, AN OUTPUT POWER TRANSFORMER INCLUDING A PRIMARY WINDING CONNECTED IN SERIES WITH A CHOKE COIL, SAID SERIES CONNECTED WINDING AND COIL BEING CONNECTED IN PARALLEL WITH SAID PRIMARY WINDING OF THE FEEDBACK TRANSFORMER, AND AN OUTPUT CIRCUIT INCLUDING THE SECONDARY WINDING OF SAID POWER TRANSFORMER, A STORAGE CONDENSER CONNECTED TO BE CHARGED BY SAID SECONDARY WINDING OF THE POWER TRANSFORMER AND MEANS FOR INTERMITTENTLY DISCHARGING SAID STORAGE CONDENSER. 